Areas with possible fire or explosion risks due to explosive atmospheres and/or mixtures - are called hazardous (or classified) locations or areas. These areas are in North America (United States and Canada) historically classified with the Class/Division system. In Europe and the rest of the world - but also more and more in North America - the Zone system is used.

The hazardous area classification system determines required protection techniques and methods for electrical installations in the location.

Class/Division System

The Class/Division/Group system is based on Article 500 of the National Electrical Code (NEC) where

Classes - defines the general nature of the hazardous material in the surrounding atmosphere

Divisions - defines the probability of hazardous material being present the surrounding atmosphere

Groups - defines the type of the hazardous material in the surrounding atmosphere

Class

Class defines the general nature (or properties) of the hazardous material in the surrounding atmosphere.

Class

Nature of Hazardous Material

Class I

Hazardous because flammable gases or vapors are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures.

Class II

Hazardous because combustible or conductive dusts are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures.

Class III

Hazardous because ignitable fibers or flyings are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures.

Division

Division defines the probability of the hazardous material being present in an ignitable concentration in the surrounding atmosphere.

Division

Probability of Hazardous Material

Division 1

The substance referred to by class has a high probability of producing an explosive or ignitable mixture due to it being present continuously, intermittently, or periodically or fromthe equipment itself under normal operating conditions.

Division 2

The substance referred to by class has a low probability of producing an explosive or ignitable mixture and is present only during abnormal conditions for a short period of time - such as a container failure or system breakdown

Group

Group defines the type of the hazardous material in the surrounding atmosphere.

Group

Type of Hazardous Material

Group A

Atmosphere containing acetylene.

Group B

Atmosphere containing a flammable gas, a flammable liquid produced vapor, or a combustible liquid produced vapor mixed with air that may burn or explode, having either a MESG (Maximum Experimental Safe Gap)1) value less than or equal to 0.45 mm or a MIC (Minimum Igniting Current)2) ratio less than or equal to 0.40 - such as hydrogen or fuel and combustible process gases containing more than 30% hydrogen by volume - or gases of equivalent hazard such as butadiene, ethyleneoxide, propylene oxide and acrolein.

Group C

Atmosphere containing a flammable gas, a flammable liquid produced vapor or a combustible liquid-produced vapor whose MESG is greater than 0.75 mm or MIC ratio is greater than 0.40 and less than 0.80 - such as carbon monoxide,ether, hydrogen sulfide, morphline, cyclopropane, ethyl, isoprene, acetaldhyde and ethylene or gases of equivalent hazard.

Group D

Atmosphere containing flammable gas, flammable liquid produced vapor, or combustible liquid produced vapor mixed with air that may burn or explode, having either a MESG value greater than 0.75 mm or a MIC ratio greater than 0.80 - such as gasoline, acetone, ammonia, benzene, butane, ethanol, hexane, methanol, methane, vinyl chloride, natural gas, naphtha, propane or gases of equivalent hazard.

Atmosphere containing carbonaceous dusts, carbon black, coal black, charcoal, coal or coke dusts that have more than 8% total entrapped volatiles or dusts that have been sesitized by other materials so they present an explosion hazard.

Group G

Atmosphere containing combustible dust not included in Group E & F - such as flour, grain, starch, sugar, wood, plastics and chemicals.

1) MESG (Maximum Experimental Safe Gap) - The maximum clearance between two parallel metal surfaces that has been found under specified test conditions to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same gas or vapor at the same concentration.

2) MIC (Minimum Igniting Current) Ratio - The ratio of the minimum current required from an inductive spark discharge to ignite the most easily ignitable mixture of a gas or vapor, divided by the minimum current required from an inductive spark discharge to ignite methane under the same test conditions.

Groups A, B, C, and D are for gases (Class I only). Groups E, F, and G are for dusts and flyings (Class II or III).

The specific hazardous materials within each group and their automatic ignition temperatures can be found in Article 500 of the National Electrical Code and in NFPA 497.

Zones - defines the general nature (or properties) of the hazardous material - if its gas or dust, and the probability of the hazardous material in the surrounding atmosphere

Groups - defines the type of the hazardous material and (partly) the location of the surrounding atmosphere

Zone

Zone defines the general nature - if it is a gas or dust - and the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere. The Zone system has three levels of hazard for gas or dust where the Division system has two.

Gases, Vapors and Mists

Article 505 National Electrical Code (NEC)

Zone

Nature and Probability of Hazard Material

Zone 0

Ignitable concentrations of flammable gases or vapors which are present continuously or for long periods of time.

Zone 1

Ignitable concentrations of flammable gases or vapors which are likely to occur under normal operating conditions.

Zone 2

Ignitable concentrations of flammable gases or vapors which are not likely to occur under normal operating conditions and do so only for a short period of time.

Dusts

Article 506 National Electrical Code (NEC)

Zone

Nature and Probability of Hazard Material

Zone 20

An area where combustible dusts or ignitable fibers and flyings are present continuously or for long periods of time.

Zone 21

An area where combustible dusts or ignitable fibers and flyings are likely to occur under normal operating conditions.

Zone 22

An area where combustible dusts or ignitable fibers and flyings are not likely to occur under normal operating conditions and do so only for a short period of time.

Zones compares to classes and divisions in the Class/Division system.

Group

Group define the type of hazardous material and (partly) the location of the surrounding atmosphere. Group is divided in three groups where Group I is reserved for mining locations. Group II is for explosive gases (Zone 0, 1 and 2) and Group III is for explosive dusts (Zone 20, 21 and 22).

Group

Type of Hazardous Material and Location of Atmosphere

Group I

Minessusceptible to firedamp (flammable mixture of gases naturally occurring in a mine).

Group II

Explosive gas atmosphere other than mines susceptible to firedamp. Group II equipment is subdivided into three subgroups.

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